This returns true if the widget has focus, or if one of its descendant
widgets does. Use this in StackWidget and TabWidget.
This also fixes HackStudio crashing on startup in StackWidget, due to
running before the window has a focused widget.
This one is a bit sketchy. While a window is inactive, none of its
widgets are considered focused (Widget::is_focused() will return false)
but this caused programmatic changes of the active widget in a tab
or stack widget to fail focus propagation from old child to new child.
Work around this by checking against Window::focused_widget() directly
instead of asking Widget::is_focused().
